1. Field of the Invention
The invention relates to a process for the preparation of a cyclobutanone of the general formula I ##STR1## or of a cyclobutenone of the general formula II ##STR2## in which formulas each A represents a halogen atom having an atomic number of not more than 35, a hydrogen atom or a substituted or unsubstituted hydrocarbyl group.
2. Description of the Prior Art
The alpha-halocyclobutanones mentioned in British patent specification No. 1,194,604 are obtained by (2+2)cycloaddition of haloketenes to olefins. This cycloaddition may be effected as described in J.A.C.S. 87 (1965) 5257-9 and Tetrahedron Letters No. 1, pp. 135-9 (1966), by dehydrochlorination of dichloroacetyl chloride with triethylamine. The dichloroketene, formed in situ, reacts with an olefin to give an alpha-chlorocyclobutanone, which, however, is obtained in a low yield, because it reacts with triethylamine with formation of a quaternary ammonium chloride.
The cycloaddition may also be effected as described in J. Org. Chem. 31 (1966) 626-8, by dehalogenation of an alpha-haloacetyl bromide with zinc dust in an inert solvent. The latter article describes the preparation of dichloroketene only, starting from trichloroacetyl bromide and zinc. The dichloroketene is isolated in hydrocarbon solvents such as hexane or octane and the solutions thus obtained are used as source of dichloroketenes (confer the said British patent specification, stating that after undesirable by-products have been removed from the solution of the haloketene the latter is reacted with an olefin). This article fails to reveal anything about the isolation of a monohaloketene prepared from a dihaloacetyl halide and zinc, which suggests that this isolation is impossible, a monohaloketene being an unstable compound, which polymerizes readily, even at very low temperature. This suggestion is fortified by Synthesis, August 1971, pp. 415-22, stating that when monochloroketene is prepared by dehydrohalogenation of monochloroacetylchloride with triethylamine in the presence of chloral, a mixture of cis-and trans-4-trichloro-methyl-2-oxetanones is produced, whilst the preparation of monochloroketene by the dehalogenation of dichloroacetyl chloride with zinc in the presence of chloral yields only alpha,beta-dichlorovinyl dichloroacetate. Consequently, the dehydrohalogenation and dehalogenation methods are not equivalent.
Applicant has indeed found that isolation of monochloroketene is impossible, because dichloroacetyl chloride and zinc in diethyl ether form a reaction mixture which does not contain any monochloroketene and which, after removal of the zinc, fails to give any cyclobutanone upon addition of 2,3-dimethyl-2-butene. Surprisingly, it has been found that monochloroketene formed in situ by dehalogenation is capable of entering into a (2+2) cyclo-addition.